(1) Field of the Invention
The present invention relates to a developing unit mounted on an image forming apparatus using electrophotography, in particular relating to an agitating and conveying device for conveying the developer to the developing area while agitating toner and carrier in a developing hopper provided for the developing unit.
(2) Description of the Prior Art
In image forming apparatuses using electrophotography such as copiers, facsimile machines, printers, so-called multi-functional machines having these functions and the like, a robust recording image can be formed by supplying toner as a component of the developer from a developing unit to the electrostatic latent image formed on the photoreceptor drum surface to form a visible toner image, transferring the toner image to a recording paper etc., as a transfer medium; and fixing the thus transferred toner image onto the recording paper.
With recent improvement in office work efficiency using image forming apparatus such as copiers, printers etc., there has been a demand for an image forming apparatus which is improved in image processing capacity, for example, in the processing speed of image forming, represented by the number of recording sheets per unit time that can be image-formed.
In the image forming apparatus having a high processing speed of image forming (high-speed machines), it is demanded that the life of the developer should be longer than that for the image forming apparatus having a medium processing speed of image forming (medium-speed and low-speed machines). The reason is that the developer or more precisely the toner is consumed in a greater amount and exposed to stress during agitation and conveyance in a higher degree compared to that of the medium and low-speed machines because a higher number of images are formed per unit time in the high-speed machine. As a result, the developer having a comparable lifetime to that of the developer used in the medium and low-speed machines will finish its life quickly, increasing the frequency of developer-related maintenance.
As one countermeasure against this problem, there is an idea to make the developing unit large. Because enlargement of the developing unit and increase in the amount of developer stored therein make it possible to deal with mass consumption of the developer in the high-speed machine, reduce the frequency with which the developer is exposed to the stress due to agitation and conveyance and also increase the lifetime.
However, it occurs frequently that the developing unit cannot be made larger in size due to structural limitations of the apparatus. Further, if the developing unit is made greater to some slight degree, it is impossible for a high-speed machine which uses a system in which toner is supplied to the developing hopper that confronts the developer bearing member, to secure long enough time to sufficiently agitate and supply uniform developer to the developer bearing member, hence it is difficult to control the toner concentration in the developer.
To deal with this, as a countermeasure against this problem, there is a proposal that the developing hopper of the developing unit is formed longer than the longitudinal length of the developer bearing member while a partitioning plate is provided between the developing area where it opposes the developing support and the portion extended longitudinally beyond the developing area so as to form an agitating area to secure uniform agitation in the agitating area.
However, even with the agitating area provided, if toner supply is performed from a position which cannot afford long enough time for agitation, the toner concentration along the longitudinal direction of the developing unit is prone to become uneven, giving rise to the problem of being unable to control the toner concentration with precision.
Further, as a method of solving the density unevenness problem due to agitation failure, patent document 1 (Japanese Patent Application Laid-open 2005-292301) discloses a configuration in which a scoop-assisting member made up of a magnetic substance is disposed between a developer bearing member and a screw-like agitator.
However, in order to obtain uniform enough images it is necessary to sufficiently agitate and mix the toner and carrier before bringing up the developer to the developer bearing member. If the scooping performance of the developer to the developer bearing member is enhanced with insufficient agitation of the developer, it is impossible to obtain homogeneous developer because the developer can be little agitated on the developer bearing member. This tendency is particularly conspicuous especially in the high-speed machines in which the holding time of the supplied developer in the developing hopper is short.